1. Field of the Invention
The present invention relates to an image pickup apparatus that drives a focus lens to perform focus detection.
2. Description of the Related Art
A contrast autofocusing (AF) method has been conventionally known as a method of moving the position of a focus lens in an image pickup apparatus such as a digital still camera to focus on an object. The contrast AF method generates a focus evaluation value corresponding to the contrast of an image using an image signal from the image pickup element. The method performs an automatic focusing operation that acquires the focus evaluation value while moving the position of the focus lens to find an in-focus position that is the focus lens position at which the focus evaluation value reaches a peak.
In the contrast AF method, a focus speed needs to be set such that a sufficient number of the focus evaluation values for the peak determination are acquired when the focus lens is in the vicinity of the in-focus position. In contrast at a position distant from the in-focus position, the focus lens is moved fast to achieve a fast AF operation. For this, it is known that the focus speed is varied based on the focus evaluation value while the focus lens is driven. Japanese Patent Laid-open No. H7-7650 discloses an automatic focus video camera that calculates the rate of change in the evaluation value that is the ratio between the amount of change in the focus evaluation value and the amount of change in the lens position, and the video camera varies the change speed of the lens position depending on whether the rate of change in the evaluation value is increasing or decreasing. Japanese Patent Laid-open No. H8-29667 discloses an automatic focusing method of predicting the in-focus position (the peak position) based on the focus evaluation value and, in a case of a distant predicted in-focus position, moving the lens fast before setting a low speed.
When a velocity control is performed after determination that the focus lens is in the vicinity of the in-focus position of an object, however, such an AF method that acquires the focus evaluation value while driving the focus lens generates a time lag from the determination to the velocity control. The shape of the focus evaluation value varies with the contrast of the object, frequency, and settings of a stop, exposure, and the like. For this reason, when the peak position of the object is determined based on the shape of the focus evaluation value, the velocity is potentially unable to decrease to a predetermined velocity that allows a sampling interval ensuring an AF accuracy before the focus lens reaches the vicinity of the in-focus position. To solve this, it is suggested to perform scanning again (rescanning) when the focus lens fails to sufficiently slow down in the vicinity of the in-focus position, but the rescanning results in a longer time for focusing.
The configurations disclosed in Japanese Patent Laid-open No. H7-7650 and Japanese Patent Laid-open No. H8-29667 are not based on consideration of the time lag from the determination of the shape of the focus evaluation value to the velocity control.